Method and apparatus for inspecting slight defects in a photomask pattern

ABSTRACT

A photomask defect inspection method is provided by which defects of pin holes with the diameter equal to or less than 0.35 μm can be detected with certainty. According to the inspection method, a pattern whose image is projected onto an imaging position by the use of illumination light (P1) for exposure consists of light transmitting portions (41) formed on a glass base (2) and light intercepting portions (42) which transmit part of the illumination light (P1) in such a way that a phase of the part of the illumination light (P1) passing through the light intercepting portions (42) is delayed with respect to a phase of the illumination light (P1) passing through the light transmitting portions (41). Slight defects in the photomask pattern are detected on the basis of a signal obtained by illuminating the pattern with inspection light having an inspection wavelength in which the transmittance (T) of the light intercepting portions (42) is defined in the following formula on the basis of a signal detection limit (Thr). When the signal detection limit (Thr) of an inspection circuit is calculated on the supposition that a signal level of the inspection light passing through the light transmitting portions (41) is equal to 1, the relational expression is T≧(Thr-0.01) 1/1 ,8.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method of and apparatus for inspectingslight defects in a photomask pattern.

2. Description of the Prior Art

A photomask 1 is used in the production of semiconductor integratedcircuits. As shown in FIG. 1, the photomask I has a transparent base 2on which, for example, two chips 3 each size of which is 10 mm×20 mm areformed. A circuit pattern 4 of a light intercepting film made ofchromium (Cr) or the like is formed in the chips 3 in high density. Thewidth of the circuit pattern 4 is, for example, 1 μm to 3 μm.

As shown enlargedly in FIG. 2, cases occur in which a part of thecircuit pattern 4 has defects, such as a pin hole 5 or a projection 8,or has flaws, such as a crack 6 or a nick 7, or has foreign substances.If exposure is performed using such a defective photomask 1, a circuitpattern different from a predetermined circuit pattern 4 is formed on asemiconductor substrate (i.e., a wafer). In other words, a semiconductorintegrated circuit having pattern defects is formed. For this reason, aninspection of whether the formed photomask 1 has defects is beforehandundergone.

There are various kinds of methods of inspecting defects in the patternof the photomask 1. Typically, an adjacent-pattern comparison method anda design-data comparison method are well known.

1 Adjacent-Pattern Comparison Method

According to this method, two adjacent chips 3 are compared with eachother and, when disagreements therebetween are found, it is judged thatdefects exist. This method is followed on the supposition that there islittle probability that two adjacent chips 3 have the same defects inthe same circuit patterns of the chips 3.

2 Design-Data Comparison Method

According to this method, a circuit pattern is observed by a defectinspection apparatus, and the observed positions are compared withdesign-data corresponding to the positions.

These defect inspection methods 1 and 2 are properly used depending onpurposes and uses.

FIG. 3 shows an example of the apparatus for inspecting defects in thepattern of the photomask 1. This inspection apparatus comprises a dataprocessing system which includes a CPU 10, a magnetic disk unit 11, amagnetic tape unit 12, a floppy disk drive unit 13, a console CRT 14, apattern monitor 15, a magnetic card unit 16, a miniprinter 17, anRS-232C adapter 18, and the like, a detective optical system whichincludes an autoloader control circuit 19, a table control circuit 20,an X-motor M1, a Y-motor M2, a θ-motor M3, an autofocus control circuit21, a piezo-element 21a, a positioning circuit 22, a control circuit 22'of, for example, a laser length measuring system, a bit developingcircuit 23, a pattern comparative inspection circuit such as a datacomparison circuit 24, an autoloader 25 accommodating various kinds ofphotomasks 1, an illumination light source 26, an illumination fielddiaphragm 27, a condenser lens 28, an X-Y table 29, an objective lens30, a photodiode array 31, a sensor circuit 32, and the like, and anobserving scope which includes reflection mirrors 33, 34, an eyepiece35, and the like (see a reference entitled "Mask Defect InspectionMethod by Database Comparison with 0.25-0.35 μm Sensitivity", in Jpn. J.Appl. Phys. Vol 33(1994)7156) . As shown in FIG. 4(a), the width ofabout 300 μm of the photomask 1 is observed by the photodiode array 31.The photodiode array 31 is disposed at a position where the circuitpattern is imaged. The photomask 1 is mounted on the X-Y table 29 and isilluminated with light from the illumination light source 26.

As shown in FIG. 4(b), the X-Y table 29 is transferred in the directionof arrow A1 at intervals of a predetermined pitch p. When measurementsin the direction of arrow A1 are completed, the X-Y table 29 istransferred by the width W in the direction of arrow A2 and thereafterthe X-Y table 29 is transferred at intervals of the predetermined pitchp in the direction of arrow A3. In the same manner, the X-Y table 29 istransferred successively in the directions of arrows A4, A5 . . . so asto inspect the whole range of the photomask 1.

The autofocus control circuit 21 drives autofocusably the objective lens30 in the axial direction of the objective lens 30 so as to keep adistance between the objective lens 30 and the photodiode array 31constant, and thereby accurate data can be obtained. The θ-motor M3controls the X-Y table 29 to keep the photomask 1 parallel to thephotodiode array 31.

Graphic data is beforehand stored as a circuit pattern in the magneticdisk unit 11. The circuit pattern 4 of the photomask 1 is projectedenlargedly onto the photodiode array 31 by means of the objective lens30, and an image of the circuit pattern 4 is formed on the photodiodearray 31. The image of the circuit pattern 4 is photoelectricallytransferred by the photodiode array 31 and is output to the sensorcircuit 32 in the form of measured data. The measured data is convertedfrom an analog signal to a digital signal and is input to a first inputterminal of the data comparison circuit 24.

On the other hand, the graphic data is transmitted to the bit developingcircuit 23 in accordance with a detected output of the positioningcircuit 22. The graphic data is converted into a binary number system bymeans of the bit developing circuit 23 and is transmitted to a secondinput terminal of the data comparison circuit 24. The output of thepositioning circuit 22 is input to a third input terminal of the datacomparison circuit 24. The data comparison circuit 24 processes thebinary bit pattern data through proper filters and thereby converts thebinary bit pattern data into a multivalue system.

The reason why the binary bit pattern data is processed through theproper filters is that the measured data is being filtered by theresolution characteristic of the objective lens 30 and the apertureeffect of the photodiode array 31.

Data in an observed position is compared with data in a correspondingposition of pattern design data in accordance with a predeterminedalgorithm by means of the data comparison circuit 24. Thereby,disagreeing positions between the design data and the measured data areregarded as defects. In this type of pattern comparative defectinspection, in order to detect slight defects, the resolution of anoptical system of an inspection means is enhanced, the algorithm forcomparison is improved, or the processing of measured signals isimproved.

However, the detection sensitivity to pattern defects largely dependsupon the kinds of the defects. Especially, if a pattern defect of thecircuit pattern 4 is a pin hole as shown in FIG. 2, it is difficult todetect it, and it is almost impossible to detect the defect of apin-hole less than 0.35 μm in diameter.

In recent years, a phase shift type of photomask shown in FIG. 6(a) hasbeen used instead of a conventional amplitude type shown in FIG. 5(a).In the amplitude type of photomask, illumination light P1 is completelyintercepted by light intercepting portions 36 made of chromium (Cr), asshown in FIG. 5(a). The illumination light P1 which has passed onlythrough light transmitting portions 37 is guided to the photodiode array31, and a circuit pattern is then imaged on the photodiode array 31 inaccordance with the amplitude intensity of light. The luminous intensitydistribution of a circuit pattern image at an imaging position 38 isshown in FIG. 5(b), where reference numeral 36' denotes a position of anintercepted image corresponding to the light intercepting portions 36,reference numeral 37' denotes a position of a transmitted imagecorresponding to the light transmitting portions 37, and referencenumeral 39 denotes a luminous intensity distribution of the circuitpattern image at the imaging position 38.

In the conventional amplitude type of photomask 1, in order to enhancethe detection sensitivity to slight defects in the circuit pattern 4,the light amplitude intensity of the circuit pattern image of thephotomask I which is formed on the photodiode array 31 is heightened tothe utmost. In other words, in order to heighten the resolution, thewavelength λ of the illumination light P1 with which the photomask 1 isilluminated is shortened, and the numerical aperture NA of the objectivelens 30 is enlarged. This is based on the optical theory that, ifilluminating conditions are fixed, the optical intensity of an imagebecomes larger as the value λ/NA becomes smaller.

The photomask 1 which has been regarded as having no defects in thecircuit pattern 4 is attached to an exposure unit. The circuit pattern 4is then imaged on a wafer by the illumination light P1 of the exposureunit having an objective lens with a large numerical aperture NA.However, it is unallowable to make the value λ/NA smaller than apredetermined value, for the following reason.

A resist serving as a photosensitive agent is applied to the wafer. Thefilm thickness of the resist is 1 μm and over, as a result ofconsidering the etching of a ground after exposure. A depth of focusequal to or larger than 1 μm is required to, with respect to thedirection of the film thickness, expose the resist to light whilekeeping the contours of the image clear.

However, the depth of focus, the wavelength λ, and the numericalaperture NA have a relationship to each other in that the depth of focusbecomes smaller in proportion to λ/(NA)². Especially, the numericalaperture NA contributes to the depth of focus by the square of thenumerical aperture NA. The limited value of the depth of focus isapproximately 0.6 μm. Thus, the conventional exposure method is limitedin enhancing the resolution of a circuit pattern image.

Consequently, the phase-shift photomask 40 (e.g., attenuated photomask)shown in FIGS. 6(a) and 6(b) has been used to obtain higher resolutionthan hitherto by the use of the conventional exposure unit.

The structure of the phase-shift photomask 40 will now be described.

As shown in FIG. 6(a), on a glass base, light intercepting portions 42are formed which are made of a material having a higher refractive indexthan that of light transmitting portions 41. The light interceptingportions 42 transmit part of the illumination light P1. A phase of thepart of the illumination light P1 which has passed through the lightintercepting portions 42 is delayed with respect to that of theillumination light P1 which has passed through the light transmittingportions 41.

The phase difference between the illumination light P1 which has passedthrough the light transmitting portions 41 and the illumination light P1which has passed through the light intercepting portions 42 causesinterference therebetween. As a result, a circuit pattern image at theimaging position 38 is formed not only by the light amplitude intensitybut also by the interference caused by the phase difference.

FIG. 6(b) shows the luminous intensity distribution of the circuitpattern image at the imaging position 38. In FIG. 6(b), referencenumeral 41' denotes a transmission image position corresponding to thelight transmitting portion 41, reference numeral 42' denotes aninterception image position corresponding to the light interceptingportion 42, and reference numeral 43 denotes a distribution of theluminous intensity of the circuit pattern image at the imaging position38.

According to the photomask 40, the minimum value δ of a luminousintensity distribution 43 can be made smaller than the minimum value δ'of a luminous intensity distribution 39. As a consequence, the contrastof the circuit pattern image having a wavelength equal to or shorterthan the wavelength λ of the illumination light P1 can be expected to beimproved. Thus, the contours of the circuit pattern image become clear.Since the resist applied to the wafer has the property of strengtheninga contrast, this effect can be heightened even more.

However, in the phase-shift photomask 40, part of light can pass throughthe light intercepting portions 42. This makes it more difficult todetect pattern defects, such as a pin hole with a diameter below 0.35μm, if inspection is carried out with inspection light same inwavelength as exposure light.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a methodof and an apparatus for inspecting slight defects in a pattern of aphotomask, by which slight defects, such as a pin hole with a diameterof 0.35 μm and less, can be inspected closely and with certainty.

In a method of inspecting slight defects in a pattern of a photomaskaccording to an aspect of the present invention, the pattern whose imageis projected onto an imaging position by using illumination light withan exposure wavelength for exposure comprises light transmittingportions formed on a transparent base and light intercepting portionsformed on the transparent base which transmit part of the illuminationlight a phase of which is delayed with respect to a phase of theillumination light passing through the light transmitting portions. Themethod includes the step of detecting defects in the pattern on thebasis of a signal obtained by illuminating the pattern with inspectionlight having a wavelength different from the exposure wavelength. Theinspection light satisfies the formula

    T≧(Thr-0.01).sup.1/1.8

where T is a transmittance of the light intercepting portion withrespect to the inspection light with the inspection wavelength, and Thris a signal detection limit of an inspection circuit, on the suppositionthat a signal level of the inspection light passing through the lighttransmitting portions is 1.

According to the present invention, a photomask is illuminated withillumination light having a longer wavelength than that of exposurelight when inspection is carried out.

The phase of the illumination light which passes through lightintercepting portions is delayed with respect to the phase of theillumination light which passes through light transmitting portions.When inspection is carried out using the illumination light having alonger wavelength than that of exposure light, a luminous intensitydistribution of the illumination light is largely varied if slightdefects in a pattern exist.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially sectional view of a photomask.

FIG. 2 is a partially enlarged view of the photomask, showing an exampleof defects in a circuit pattern in a chip formed in the photomask.

FIG. 3 is a descriptive drawing of an apparatus for inspecting defectsin a pattern of a photomask.

FIGS. 4(a) and 4(b) show a relationship between a photodiode array shownin FIG. 3 and the photomask. FIG. 4(a) is a plan view of the photodiodearray, and FIG. 4(b) is a perspective view of an X-Y table where thephotomask is mounted.

FIGS. 5(a) and 5(b) show an example of amplitude-type photomasks. FIG.5(a) is a sectional view of the amplitude-type photomask, and FIG. 5(b)shows a luminous intensity distribution at an imaging position.

FIGS. 6(a) and 6(b) show an example of phase-shift photomasks. FIG. 6(a)is a sectional view of the phase-shift photomask, and FIG. 6(b) is aluminous intensity distribution at an imaging position.

FIG. 7 is a graph showing a difference in the luminous intensitydistribution depending on the site of a pattern defect.

FIG. 8 is a graph showing the variation of a luminous intensity when thesize of the pattern defect is varied.

FIGS. 9(a) to 9(h) are graphs showing the variation of a luminousintensity distribution when the size of the pattern defect is variedusing illumination light which serves as inspection light and has alonger wavelength than a wavelength of exposure light.

FIG. 10 is a graph showing differences in output. FIG. 11 is a graphshowing a relationship between a difference in output and a detectionlimit.

FIG. 12 is a graph showing a relationship between the size of a patterndefect and differences in output when a transmittance of a lightintercepting portion is 50%.

FIG. 13 is a graph showing a relationship between the size of thepattern defect and the differences in output when the phase differenceis varied at intervals of 0.2π from 0.3π to 0.9π on the condition thatthe transmittance of the light intercepting portion is 10%.

FIG. 14 is a graph showing a relationship between the size of thepattern defect and the differences in output when the phase differenceis varied at intervals of 0.2π from 0.3π to 0.9π on the condition thatthe transmittance of the light intercepting portion is 20%.

FIG. 15 is a graph showing a relationship between the size of thepattern defect and the differences in output when the phase differenceis varied at intervals of 0.2π from 0.3π to 0.9π on the condition thatthe transmittance of the light intercepting portion is 30%.

FIG. 16 is a graph showing a relationship between the size of thepattern defect and the differences in output when the phase differenceis varied at intervals of 0.2π from 0.3π to 0.9π on the condition thatthe transmittance of the light intercepting portion is 40%.

FIG. 17 is a graph showing a relationship between the size of thepattern defect and the differences in output when the phase differenceis varied at intervals of 0.2π from 0.3π to 0.9π on the condition thatthe transmittance of the light intercepting portion is 50%.

FIG. 18 is a graph showing a relationship between the size of thepattern defect and the differences in output when the phase differenceis varied at intervals of 0.2 from 0.3π to 0.9π on the condition thatthe transmittance of the light intercepting portion is 60%.

FIG. 19 is a graph showing a relationship between the size of thepattern defect and the differences in output when the phase differenceis varied at intervals of 0.2π from 0.3π to 0.9π on the condition thatthe transmittance of the light intercepting portion is 70%.

FIG. 20 is a graph showing how to ascertain whether pattern defects canbe detected according to differences in output.

FIG. 21 is a graph showing how to ascertain whether pattern defects canbe detected according to differences in output.

FIG. 22 is a graph showing how to ascertain whether pattern defects canbe detected according to differences in output.

FIG. 23 is a graph showing a relationship between a signal detectionlimit Thr and the minimum value of the transmittance T in the lightintercepting portions at which the difference |Ss-Bs| on a level withthe signal detection limit Thr is obtained when the phase difference isπ.

FIG. 24 is a graph showing isoplethic curves of the signal detectionlimit Thr which are calculated according to Table 1.

FIG. 25 is a graph showing isoplethic lines Q of the signal detectionlimit Thr which are obtained by transforming the phase difference φ intoa sine formula.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Generally, a phase-shift photomask 40 is designed to meet the followingtwo conditions. One of them is that the transmittance of light waves inlight intercepting portions 42 is 1 to 4% in a case where the wavelengthof illumination light P1 for exposure is an exposure wavelength λ, andthe other is that the phase of the illumination light P1 transmitted bythe light intercepting portions 42 has a phase lag of π with respect tothe phase of the illumination light P1 transmitted by light transmittingportions 41. Consideration will be given to a case where defects in acircuit pattern of the phase-shift photomask 40 are inspected using apattern defect inspection apparatus shown in FIG. 3.

The minimum dimensions of the circuit pattern of the phase-shiftphotomask 40 are larger than a resolution limit in a detection opticalsystem. An image of the circuit pattern thrown on a photodiode array 31retains the configuration of the circuit pattern. In contrast, patterndefects, such as an extraneous substance or a flaw, are much larger andsmaller in dimensions than the pattern. An image of a large patterndefect is thrown onto the photodiode array 31 while retaining theconfiguration corresponding to the pattern defect.

Therefore, a luminous intensity distribution 44 (an image) becomes theconfiguration equivalent to the large pattern defect (see FIG. 7). Whenan image based on a small pattern defect becomes below the resolutionlimit, the configuration corresponding to the small pattern defectcannot be retained. In other words) the image based on the small patterndefect becomes a spot image which is determined by the resolution in thedetection optical system. Reference numeral 45 in FIG. 7 denotes aluminous intensity distribution formed by pattern defects having thedimensions smaller than the resolution limit. We will discuss not largepattern defects but small pattern defects because the large patterndefects are possible to detect with sufficient ease by the conventionalpattern defect inspection method.

As shown in FIG. 8, the luminous intensity 46 which reaches thephotodiode array 31 varies according to the size of the pattern defect.In principle, pattern defects can be detected when the luminousintensity whose value of S/N is equal to or greater than 1 reaches thephotodiode array 31. Herein, the noise of the photodiode array 31 isdenoted by reference character N and a photoelectric transformationsignal of the light which has reached the photodiode array 31 by S. Theminimum size Q of a detectable pattern defect equates with the size of apattern defect which can obtain a luminous intensity equivalent toS/N=1.

The refractive index and transmittance of a high refractive substanceout of which the light intercepting portions 42 are made are variedaccording to the variation of the wavelength of the illumination lightP1. If an inspection wavelength λ0 greater than the exposure wavelengthλ is selected properly, the transmittance at which the illuminationlight P1 used as inspection light is transmitted by the lightintercepting portions 42 can be made higher than the transmittance inthe exposure wavelength λ. Silicon nitride (SiN), molybdenum siliside(MoSi), silicon carbide (SiC), or the like is used as the highrefractive substance out of which the light intercepting portions 42 aremade. For example, the transmittance at which the illumination light P1used as the inspection light is transmitted in the wavelength λ0 by thelight intercepting portions 42 is designed to become 50%, and the phasedifference between the illumination light P1 transmitted by the lighttransmitting portions 41 and the illumination light P1 transmitted bythe light intercepting portions 42 is designed to become π. As shown inFIGS. 9(a) to 9(h), the luminous intensity distribution 47 varies whenthe diameter of a pin hole 5 is successively varied. In FIGS. 9(a) to9(h), reference numeral 48 is regarded as a luminous intensity (a baseoutput in a case where photoelectric transformation is carried out bythe photodiode array 31) obtained by the illumination light P1 which istransmitted by the light intercepting portions 42, reference numeral 49is regarded as the luminous intensity obtained by the illumination lightP1 which is transmitted mainly by the pin hole 5, and reference numeral50 is regarded as a luminous intensity obtained by the interferencebetween the illumination light P1 which is transmitted by the lightintercepting portions 42 and the illumination light P1 which istransmitted mainly by the pin hole 5.

As shown in FIG. 10, a base output based on the illumination light P1which is transmitted by the light intercepting portions 42 is designatedby reference character Bs, the maximum value of the detected outputwhich is larger than the base output Bs is designated by Sb, and theminimum value of the detected output which is smaller than the baseoutput Bs is designated by Ss. From the luminous intensity distribution47 shown in FIGS. 9(a) to 9(h), there are calculated the absolute value|Sb-Bs| of a difference between the detected output maximum value Sb andthe base output Bs in the inspection wavelength λ0, and the absolutevalue |Ss-Bs| of a difference between the detected output minimum valueSs and the base output Bs in the inspection wavelength λ0. There arealso calculated the absolute value |Sb-Bs| of a difference between thedetected output maximum value Sb and the base output Bs in the exposurewavelength A, and the absolute value |Ss-Bs|' of a difference betweenthe detected output minimum value Ss and the base output BS in theexposure wavelength λ. The calculation results are plotted into curvedlines to obtain a graph shown in FIG. 11, wherein a solid line denotesthe absolute value |Sb-Bs| of the difference between the detected outputmaximum value Sb and the base output Bs, and an alternate long and shortdash line denotes the absolute value |Ss-Bs| of the difference betweenthe detected output minimum value Ss and the base output Bs.

Broken lines in FIG. 11 denote the absolute value |Sb-Bs|' of thedifference between the detected output maximum value Sb and the baseoutput Bs in the exposure wavelength λ, and the absolute value |Ss-Bs|'of the difference between the detected output minimum value Ss and thebase output Bs in the exposure wavelength λ.

The absolute value |Sb-Bs| of the difference in the inspectionwavelength λ0 can be obtained as a much larger signal level than theabsolute value |Sb-Bs|' of the difference in the exposure wavelength λin the range where the pin hole 5 is small in diameter. Supposing that,as shown in FIG. 11, a signal detection limit Thr is set between theabsolute value |Sb-Bs| of the difference in the inspection wavelength λ0and the absolute value |Sb-Bs|' of the difference in the exposurewavelength λ in a range to be measured, the diameter of the pin holewhich is determined on the basis of the intersecting point between theabsolute value |Sb-Bs|' in the exposure wavelength λ and the signaldetection limit Thr is equivalent to the minimum size Q of the pin hole5 which can be detected in the exposure wavelength λ. The diameter ofthe pin hole which is determined on the basis of the intersecting pointbetween the absolute value |Ss-Bs| of the difference between thedetected output minimum value Ss and the base output Bs and the signaldetection limit Thr is equivalent to the minimum size Q' of the pin hole5 which can be detected in the case where the illumination light P1having a greater wavelength λ0 than the exposure wavelength λ is used.Thereby, the diameter of the pin hole can be made much smaller than theminimum size Q in the exposure wavelength λ.

Hence, design data which is compared with the detected outputs istransformed into difference data in consideration of the transmittanceand the phase difference in the inspection wavelength λ0. The differencebetween the detected outputs is compared with the difference data bymeans of a data comparison circuit 24 so that slight defects of apattern can be detected.

Consideration will now be given in more detail to the relationshipbetween a phase difference and a transmittance of the light interceptingportions 42 in the inspection wavelength λ0 of the illumination light P1used for the inspection of pattern defects.

FIG. 12 is a graph showing a relationship between the differences|Sb-Bs|, |Ss-Bs| and the size of a pattern defect when the phasedifference is varied from 0.3π to 1.7π on the condition that thetransmittance at which the illumination light P1 is transmitted in thewavelength λ0 by the light intercepting portions 42 is 50%. As obviouslyshown in FIG. 12, the differences are symmetrical about the centralpoint of the phase difference π (e.g., the difference at 0.3π is thesame value as that at 1.7π). The difference |Ss-Bs| becomes maximum atthe phase difference π. Therefore, slight defects can be detected inhigher probability when the phase difference is π between the phase ofthe illumination light P1 which is transmitted by the light interceptingportions 42 and that of the illumination light P1 transmitted by thelight transmitting portions 41.

FIGS. 13 to 19 are graphs resulting from varying the transmittance atwhich the illumination light P1 is transmitted in the wavelength λ0 bythe light intercepting portions 42 from 10% to 70%, respectively, andplotting the differences about each of the phase differences 0.3π, 0.5π,0.7π and 0.9π. As can be seen evidently in FIGS. 13 to 19, the higherthe transmittance becomes and/or the closer the phase difference comesto π, the larger the difference |Ss-Bs| becomes and, on the other hand,the smaller the difference |Sb-Bs| becomes.

Thus, when the difference in output is varied by the transmittanceand/or the phase difference, cases occur in which the difference |Ss-Bs|becomes equal to or less than the signal detection limit Thr in therange where the size of a slight defect is larger, depending upon theselection of the transmittance and/or the phase difference in the lightintercepting portions 42, as shown in FIG. 20. In this case, the slightpattern defect cannot be detected between α and β. As shown in FIG. 21,however, the absolute value of the difference |Ss-Bs| becomes largerthan the signal detection limit Thr when the transmittance in the lightintercepting portions 42 is made sufficiently high. In this case, theslight pattern defect can be detected merely by using the difference|Ss-Bs|. When the difference |Ss-Bs| is substantially equal to thesignal detection limit Thr because of the low transmittance in the lightintercepting portions 42, as shown in FIG. 22, the pin hole defect canbe detected by using the difference |Sb-Bs| in combination with thedifference |Ss31 Bs|.

FIG. 23 shows a relationship between the signal detection limit Thr andthe minimum value of the transmittance T in the light interceptingportions 42 which allows obtaining the difference |Ss-Bs| same in levelas the signal detection limit Thr when the phase difference is π. Acurved line in FIG. 23 is obtained from the following equation;

    T=(Thr-0.01).sup.1/1.8

Then, it is possible to detect slight defects in the pattern on thebasis of a signal obtained by illuminating the pattern with inspectionlight having an inspection wavelength different from the exposurewavelength. The inspection light satisfies the formula

    T≧(Thr-0.01).sup.1/1.8

where T is a transmittance of the light intercepting portions withrespect to the inspection light with the inspection wavelength, and Thris a signal detection limit of an inspection circuit, on the suppositionthat a signal level of the inspection light passing through the lighttransmitting portions is 1.

As can be seen in FIG. 23, the curved line is not extended to the rangewhere the signal detection limit Thr is larger than 0.3. The difference|Ss-Bs| does not exceed the signal detection limit Thr at a lower valuethan this transmittance.

The minimum values of the difference |Ss-Bs| are then calculated withrespect to the respective values of the phase difference in a case wherethe transmittance T is varied from 10% to 100%. In Table 1 (see thefollowing attached sheet), those calculated values are shown on thesupposition that the intensity of light which reaches the photodiodearray 31 is 1 when the photomask 40 is not set.

As shown in FIG. 24, isoplethic curves Q1 to Q5 of the signal detectionlimit Thr are obtained by plotting the values of Table 1 on a graph. InFIG. 24, the isoplethic curves Q1 to Q5 are drawn by tracing the plottedvalues when the signal detection limit Thr is 0.03, 0.05, 0.1, 0.2, and0.3, respectively. The difference |Ss-Bs| can be detected in theright-side area of each of the isoplethic curves Q1 to Q5.

Each line on the graph of FIG. 25 is obtained by transforming thenumerical values on an ordinate axis (i.e., the phase difference φ) intovalues on sin (φ/2) with respect to the isoplethic curves Q1, Q2, Q3,and 5 in FIG. 24. The transmittance T which is obtained by transformingthe phase difference φ into a sine formula becomes a linear relationalexpression.

From Table 1, wavelengths are selected by which the phase difference φcan satisfy the following relational expressions:

When Thr=0.03 and T<25(%),

sin (φ/2)=1.17-1.4T;

when Thr=0.03 and T≧25(%),

sin (φ/2)=1.0-0.75T;

when Thr=0.05 and T<30(%),

sin (φ/2)=1.2-1.25T;

when Thr=0.05 and T≧30(%),

sin (φ/2)=1.06-0.77T;

when Thr=0.1,

sin (φ/2)=1.19-0.81T; and

when Thr=0.3,

sin (φ/2)=1.40-0.72T.

If the phase difference obtained when the relations are solved isdesignated by φm, the phase difference φ is between (2n π+φm) and(2(n+1)π-φm), wherein reference character n denotes a positive integralnumber or 0 (zero). If the phase difference obtained by interpolation inthe transmittances is designated by φm in a case where the detectionlimit Thr is an intermediate value thereof, the phase difference φ isbetween (2nπ+φm) and (2(n+1)π-φm), wherein reference character n denotesa positive integral number or 0 (zero).

As described above, when inspecting the slight defects of the circuitpattern of the phase-shift photomask 40, use is made of inspection lighthaving a longer wavelength than an exposure wavelength. Thereby, defectsof a pin hole with the diameter below 0.35 μm can be detected withcertainty. It is desirable that the photomask 40 is made of a substancewhose transmittance increases in the range of the longer wavelength thanthe exposure wavelength λ.

For example, in the photomask 40 where the i-line (having the wavelengthof 365 nm) of a super-high-pressure mercury lamp is used as the exposurewavelength λ, it is preferable that slight defects of the circuitpattern are inspected by using visible rays of light. As anotherexample, in the photomask 40 where a KrF excimer laser (having thewavelength of 249 nm) is used as the exposure wavelength, it ispreferable that the i-line (having the wavelength of 365 nm) of asuper-high-pressure mercury lamp is used as inspection light and thatthe light intercepting portions 42 are constructed by a substance whichsatisfies the relation shown in FIG. 23 in the wavelength range of thei-line. In addition to the two examples mentioned above, in thephotomask 40 where a KrF excimer laser having the wavelength of 193 nmis used as the exposure wavelength, it is preferable that the KrFexcimer laser having the wavelength of 249 nm is used as inspectionlight and that the light intercepting portions 42 are constructed by asubstance which satisfies the relation shown in FIG. 23 in thewavelength range of the KrF excimer laser having the wavelength of 249nm.

Since this invention is constructed as described above, slight defectsin the circuit pattern of the phase-shift photomask, namely, defects ofa pin hole with the diameter of 0.35 μm or less can be detected withoutfail.

According to this method of inspecting slight defects of the photomask40, a pattern defect inspecting apparatus can be developed which cancope with the correction of chromatic aberration and a heightenednumerical aperture (N.A.), from the point of view of optical materials,it use is made of the i-line (having the wavelength of 365 nm) of asuper-high-pressure mercury lamp as the inspection wavelength λ0 ofinspection light.

In the embodiment mentioned above, there was described the method inwhich pattern defects are inspected by using the absolute value of thedifference |Ss-Bs|. However, this invention is not limited to thismethod.

For example, an output of the photodiode array 31 is differentiated and,based on the differentiation result, pattern defects are inspected. Inthis case, a differentiation value Δn in the n-th position ("-th" is asuffix designating an ordinal number) of the photodiode array 31 isobtained from the following equation:

    Differentiation value Δn=(S(n+1)-S(n-1))/d

where S(n+1) is an output in the (n+1)-th position of the photodiodearray 31, S(n-1) is an output in the (n-1)-th position thereof, and d isa pitch between picture elements. In other words, based on a differencebetween output values before and after a picture element, adifferentiation value in its intermediate position is calculated.

A method of obtaining a differentiation value Δn is not limited to theabove-mentioned method. Another method can be adopted, of course.

If the adjacent-pattern comparison method is used as the pattern defectinspection method, pattern defects can be inspected by comparing asignal obtained from a circuit pattern to be inspected with a signalobtained in a circuit pattern adjacent thereto.

Further, if the design-data comparison method is used as the patterndefect inspection method, a basic signal is generated and stored whichis obtained when an ideal circuit pattern is illuminated with inspectionlight having a wavelength different from that of exposure light, andthereafter pattern defects can be inspected by comparing a signalobtained from the circuit pattern to be inspected with the basic signal.

                                      TABLE 1                                     __________________________________________________________________________               TRANSMITTANCE T (%)                                                PHASE DIFFERENCE φ                                                                   10  20  30 40 50  60  70 80 90 100                                 __________________________________________________________________________    0.9π    0.02                                                                              0.06                                                                              0.12                                                                             0.18                                                                             0.25                                                                              0.33                                                                              0.40                                                                             0.48                                                                             0.57                                                                             0.65                                0.7π    0.01                                                                              0.03                                                                              0.07                                                                             0.12                                                                             0.17                                                                              0.24                                                                              0.30                                                                             0.37                                                                             0.45                                                                             0.53                                0.5π     0.001                                                                             0.004                                                                            0.02                                                                             0.03                                                                             0.06                                                                              0.10                                                                              0.14                                                                             0.20                                                                             0.26                                                                             0.33                                0.3π    --  --  -- --  0.001                                                                             0.005                                                                            0.02                                                                             0.04                                                                             0.08                                                                             0.13                                0.1π    --  --  -- -- --  --  -- -- -- 0.02                                __________________________________________________________________________

What is claimed is:
 1. A method of inspecting slight defects in apattern of a photomask, an image of said pattern being projected onto animaging position by using illumination light with an exposure wavelengthfor exposure, said pattern comprising light transmitting portions formedon a transparent base and light intercepting portions formed on thetransparent base which transmit part of the illumination light a phaseof which is delayed with respect to a phase of the illumination lightpassing through said light transmitting portions, said method comprisingthe step of:detecting defects in said pattern on the basis of a signalobtained by illuminating said pattern with inspection light having aninspection wavelength different from the exposure wavelength, saidinspection light satisfying the relational expression

    T≧(Thr-0.1).sup.1/1.8

where T is a transmittance of said light intercepting portions withrespect to said inspection light with the inspection wavelength, and Thris a signal detection limit of an inspection circuit, on the suppositionthat a signal level of said inspection light passing through said lighttransmitting portions is
 1. 2. The method of claim 1, wherein theinspection wavelength of said inspection light is longer than theexposure wavelength of said illumination light for exposure.
 3. Themethod of claim 1, wherein a phase difference φ between a phase of saidinspection light passing through said light transmitting portions and aphase of said inspection light passing through said light interceptingportions is selected to, in accordance with the transmittance T of saidlight intercepting portions and the signal detection limit Thr, satisfythe following relational expression:

    (2nπ+φm)≦φ≦(2(n+1)π-φm)

where n is a positive integral number including zero (0), and φm is aphase difference obtained by solving the following equations:

    sin (φ/2)=1.17-1.4T when Thr=0.03 and T<25(%);

    sin (φ/2)=1.0-0.75T when Thr=0.03 and T≧25(%);

    sin (φ/2)=1.2-1.25T when Thr=0.05 and T<30(%),

    sin (φ/2)=1.06-0.77T when Thr=0.05 and T≧30(%);

    sin (φ/2)=1.19-0.81T when Thr=0.1; and

    sin (φ/2)=1.40-0.72T when Thr=0.3;

or

    (2nπ+φm)≦φ≦(2(n+1)π-φm)

where n is a positive integral number including zero (0), and φm is aphase difference obtained by interpolation in the transmittances if thesignal detection limit Thr is an intermediate value thereof.
 4. Themethod of claim 1, comprising the step of detecting defects from adifference in output of a signal which is obtained from the photomask bymeans of the inspection light.
 5. The method of claim 1, comprising thestep of obtaining a differential output of a signal obtained from thephotomask by means of the inspection light and detecting defects in saidpattern on the basis of said differential output.
 6. The method of claim1, comprising the step of detecting defects by comparing a signalobtained from a circuit pattern which is an object to be inspected onthe photomask by means of the inspection light with a signal obtainedfrom a circuit pattern adjacent to said circuit pattern by means of theinspection light.
 7. The method of claim 1, comprising the step ofdetecting defects by comparing a signal obtained from a circuit patternwhich is an object to be inspected on the photomask by means of theinspection light with a reference signal memorized in advance, saidreference signal being obtained when an ideal photomask is illuminatedwith the inspection light.